The present invention relates generally to a light-emitting diode device using a light-emitting diode chip as a light emitting source thereof. More particularly, this invention relates to a light-emitting diode device which can be applied to a backlight apparatus for illuminating a liquid-crystal display panel from the back thereof and a backlight apparatus and a liquid-crystal apparatus using the light-emitting diode device.
In recent years, instead of a CRT (cathode-ray tube), very thin display apparatus such as a liquid-crystal display (LCD) apparatus and a plasma display panel (PDP) apparatus have been proposed so far and they are now commercially available on the market as display apparatus for use with television receivers. In particular, liquid-crystal display apparatus using liquid-crystal display panels can be driven with low power consumption and large-size liquid-crystal display panels become more inexpensive so that they are widely used progressively. Therefore, the liquid-crystal display apparatus are now technically under study and development.
In such liquid-crystal display apparatus, backlight type liquid-crystal display apparatus capable of displaying color image by surface-illuminating a transmissive liquid-crystal display panel with a color filter from the back side with a backlight apparatus become the mainstreams.
A cold cathode fluorescent lamp (CCFL) using a fluorescent tube to emit white light and a light-emitting diode (LED) are considered to be promising light sources of the backlight apparatus.
In particular, after a blue light-emitting diode has been developed, light-emitting diodes which are able to emit red light, green light and blue light of three primary colors of light become completed. Thus, when red light, green light and blue light emitted from these light-emitting diodes are mixed, it is possible to obtain white light with high color purity. Accordingly, since color purity is increased through the liquid-crystal display panel by using the light-emitting diodes as the light source of the backlight apparatus, it is possible to considerably widen a color reproduction range as compared with that of the CCFL (cold cathode fluorescent lamp). Further, it is possible to considerably increase brightness of the backlight apparatus by using a light-emitting diode (LED) with high output.
When the LED is used as the light source of the backlight apparatus as described above, if the LED is of the right-under type, that is, the LED is located right under the light emission surface, then since the LED is a point source, it is unavoidable that brightness will become ununiform and that color will become ununiform.
As a method for solving this problem, there was proposed a side-emitter type light-emitting diode device, that is, so-called LED module in which a lens surrounding an LED chip which emits light in all directions is formed as a shape to reflect light, emitted above, in the lateral direction to thereby emit light mainly in the lateral direction. See, NIKKEI ELECTRONICS (NIKKEI BUSINESS PUBLICATION INC.), Dec. 20, 2004, (No. 889), pp. 123 to 130.
Since the backlight apparatus using such side-emitter type light-emitting diode devices can easily mix respective lights of RGB (red, green and blue) and can prevent brightness and color from becoming ununiform, the above backlight apparatus is useful.
However, even when the light-emitting diode device for emitting light mainly in the lateral direction is in use as described above, it is not yet possible to completely remove light emitted right above the light-emitting diode.
FIG. 1 of the accompanying drawings is a schematic cross-sectional view showing an arrangement of a light-emitting diode device in which a lens surrounding the light-emitting diode is formed as a shape to emit light mainly in the lateral direction. FIG. 1 shows an example of a light-emitting diode device in which a light-emitting diode chip 1 is disposed on a sub-mount 2, a lens 3 which surrounds the sub-mount 2 being shaped as a convex-like curved surface protruded in the lateral direction, the upper portion of the lens 3 being shaped as a concave-like curved surface.
FIG. 2 shows a typical light radiation angle distribution in this example. As very small peaks lie within an angular extent shown by a solid line A in FIG. 2, it is to be noted that a light-emitting diode device using a related-art light-emitting diode may not avoid light from being emitted in the direction right above the lens. Light emitted in the direction right above the lens may not be mixed with light emitted from other LED with a desired ratio, which causes color to be ununiform.
The light emitted in the direction right above the LED as described above is light going straight from the LED chip along the central axis of the lens without being refracted within the lens. The reason for this is that, when a lens is manufactured by a suitable process such as injection molding, if the upper portion of the lens, for example, the acute angle portion is not accurately formed with a desired angle, then light going right above the lens may not be completely emitted in the lateral direction and thereby leaked in the upper portion.
On the other hand, there is proposed a method in which a reflection coating surface is provided right above the lens of the light-emitting diode or a method in which a transparent substrate is provided on the upper portion of the light-emitting diode, a pattern of a photo-absorption member being formed on the portion right above a light source by a suitable method such as printing to thereby decrease light emitted in the direction right above the light-emitting diode (LED) as has been described so far in the above-described Cited Non-Patent Reference 1. According to these methods, however, although red light is decreased in the portion right above the red LED, intensities of green light and blue light emitted from the surrounding light-emitting diodes (LEDs) are increased relatively and hence it is unavoidable that light of cyan, which is mixed color of green and blue, becomes conspicuous.
Therefore, even when the display apparatus has the above-mentioned arrangement in which the upper surface is shielded from the light, such arrangement is not sufficient so that it is difficult to completely remove a brilliant point caused by light leaked from substantially the central position of the light-emitting diode, that is, so-called hot spot and that it is also difficult to avoid ununiformity of brightness and ununiformity of color. In order to sufficiently decrease ununiformity of brightness and ununiformity of color, it is necessary to relatively increase a space between the light source and the display panel. Thus, it is difficult to decrease the thickness of the backlight apparatus and the liquid-crystal display apparatus. Further, depending on the materials of the reflection coating and photo-absorption material, an amount of absorbed light and a quantity of ineffective light are increased, which may cause luminous efficiency to be lowered and a utilization efficiency of light also is caused to be decreased.